This invention relates generally to the growth of single crystals, and, more particularly, to the growth of fine single crystal fibers enclosed in a glass cladding.
The atoms or molecules of many materials solidify in regular arrangments called crystals, wherein a basic structural arrangement of the atoms or molecules is repeated to form the solid. Due to the mode of growth, and possibly due to subsequent processing, the majority of crystalline materials familiar to most persons develop as polycrystals. Polycrystals are essentially islands of single-crystal material bonded together at interfaces called grain boundaries, to form a solid. Single crystals, wherein an entire solid forms with one crystalline orientation and without grain boundaries, can be prepared under carefully controlled conditions and with special attention and care.
In certain applications, single crystals offer important advantages over polycrystals in respect to the properties that can be attained. In one particular field, optics, properly oriented single crystals of small size can be used to modify the properties of the light passing through the single crystals in a manner not possible with polycrystals due to their multiplicity of crystallographic orientations and internal scattering of the light. Devices incorporating such single crystals are used in optical circuits that transmit and process light.
In one particular type of application, fine single crystals of optically nonlinear materials are the key components of optical processing devices that alter the frequency of light passed through the devices. Certain optically nonlinear single crystal materials permit frequency adding, frequency subtracting, frequency doubling, and the like. To cite one example, two frequencies of light can be added by a single crystal of KNbO.sub.3. If infrared light having a wagvelength of 0.82 micrometers, and infrared light having a wavelength of 0.83 micrometers, are simultaneously introduced into one end of the single crystal, blue light having a wavelength of 0.42 micrometers is emitted at the other. In such an application, the crystal visibly indicates a frequency shift between two beams.
The single crystals of optically nonlinear materials are made very fine in diameter, to achieve high efficiency and so that they can be built into optical processing circuits as components. The optical fiber transmission lines used in such circuits are typically very fine, on the order of 0.001 inch or less in diameter. It is therefore particularly desirable to have the optical processing components such as the single crystals of optical nonlinear materials of a similar size, so that they can be interfaced directly to the optical fibers and to other components.
The optically nonlinear materials having melting points on the order of about 100.degree. C. are typically organic single crystals. To achieve the desired results, the single crystals should have lengths many times that of their diameters, as with a length to diameter ratio of over 1000. As a result, fine fibers of such materials are easily broken, and even normal handling and use may cause breakage. Special care must therefore be taken to protect the fine fibers during growth, handling, and use.
In one technique, the fine single crystals are formed inside a hollow glass tube. Once formed, each single crystal remains inside its tube during use, so that the tube protects and supports the crystal. The composition and dimensions of the glass tube are selected so as not to interfere with the functioning of the optically active single crystal.
The single crystal is grown inside the glass tube by placing the glass tube into a vertical orientation and lowering it into a reservoir of the molten single crystal material maintained at a temperature just above the melting point of the material. The molten material rises in the tube by capillary action, and the tube is then slowly withdrawn upwardly, causing the molten material to solidify as a crystal at the emerging end of the tube. Since the molten material in the tube rises under the capillary action, the solidification of the optical crystal material inside the glass tube occurs at a point that is typically several inches above the surface of the reservoir.
To avoid premature solidification and achieve unidirectional solidification of the crystal, the molten material that has risen by capillary action must be maintained above its melting point over its entire height rise. Moreover, the temperature at the point of solidification must be maintained constant to within one degree Celsius over long periods of time. The glass tube is moved upwardly at a rate of only about one inch per hour or less, and the time required to grow a long single crystal is typically several hours. If the temperature of the solidification is permitted to vary significantly during that period, there will be lengthwise inhomogeneities grown into the crystal. Such inhomogeneities interfere with the ability of the finished crystal to perform its optical functions.
It has been the prior practice to maintain the temperature constant at the point of solidification of the crystal by drawing the glass tube through a constant temperature bath, such as an oil bath. The bath has a large thermal mass and is heated by resistance heaters. It has been thought that such a bath stabilizes the temperature at the point of solidification sufficiently to produce a good quality optical crystal.
It has now been observed that the use of a constant temperature bath of the conventional type actually creates undesirable temperature variations, since varying temperature distributions are established in the bath. The convection currents of the bath circulate hot bath liquid, causing the temperature variations. Even though the variations are relatively small, the result is that the finished crystal exhibits lengthwise inhomogeneities that are detrimental to their optical performance. Further, since the material of the constant temperature bath is selected as unrelated to the crystal being grown, there is no inherent stabilization of the bath temperature near to the melting point of the optical crystal.
There therefore exists a need for a better method and apparatus for growing fine single crystals in glass tubes. The approach should achieve a very high degree of temperature stabilization at the melting point of the crystal being grown, so that the temperature can be maintained over extended periods. The present invention fulfills this need, and further provides related advantages.